Apparatus for developing electrophotographic sheet



June 5, 1962 M. A. LACE 3,037,478

APPARATUS FOR DEVELOPING ELECTROPHOTOGRAFHIC SHEET Filed Oct. 23, 1957H; 5 H; I

2. Sheets-Sheet 1 39 INvENToR MELVIN A. LACE by: v

ATTY.

M. A: LACE 3,037,478

APPARATUS FOR DEVELOPING ELECTROPHOTOGRAPHIC SHEET June 5, 1962 2Sheets-Sheet 2 Filed Oct. 23, 1957 INVENTOR MELVIN A. LACE ATTY.

Patented June 5, 1962 3,037,478 APPARATUS FOR DEVELOPING ELECTRO-PHOTOGRAPHIC SHEET Melvin A. Lace, Chicago, Ill., assignor to AmericanPhotocopy Equipment Company, Chicago, Ill., a corporation of IllinoisFiled Oct. 23, 1957, Ser. No. 691,924 8 Claims. (Cl. 118-637) Thisinvention relates to a novel apparatus and method for magnetic brushdevelopment of latent electrostatic images on electropho-tographicsheets or plates.

In the electrophotographic process the surface of a photoconductiveinsulating material is first provided with a uniform electrostaticcharge. The charged surface of the electrophotographic sheet or plate isthen exposed to a light image, thereby discharging the charges on thoseareas of the surface irradiated by light while leaving the remainder ofthe surface in a charged state so as to form a latent electrostaticimage substantially corresponding to the light image. The latentelectrostatic image is then developed by applying a powdered toner whichadheres to the charged areas of the surface. The powder may then befixed directly to the electrophotographic sheet or may be transferred toanother surface and fixed thereon.

The so-called magnetic brush system is one of the presently preferredtechniques for developing the latent electrostatic image, that is, forapplying the toner particles to the surface of the sheet bearing theelectrostatic image. The magnetic brush comprises a magnet to which isadhered a developer mix including iron filings and toner particles heldto the magnet in the form of a fibrous mass by the magnetic field. Whenthe mass is wiped over the surface bearing the latent electrostaticimage, the toner particles are retained on the charged areas of thesheet surface by electrostatic attraction due to the electrostaticcharge developed upon the toner particles by a triboelectric reactionwith the iron filings.

It has heretofore been discovered that improved results can be obtainedby applying a bias potential or unidirectional electric field betweenthe electrostatic image and the magnetic brush during the developmentstep. The contrast of the developed image may be controlled by varyingthe voltage of the applied potential. Also, either a direct or reversedimage may be developed, as desired, in accordance with the direction ofthe applied electrostatic field.

As heretofore practiced, the electrophotographic sheet bearing theelectrostatic i age on its front surface is placed with its rear surfaceagainst a metal back-up plate, and the bias potential is applied betweenthe plate and the magnetic brush. This technique involves a seriousdisadvantage when developing the marginal areas of anelectrophotographic sheet having a width less than the width of themetal back-up plate. More specifically, when the magnetic brush contactssuch a sheet along an edge thereof so as to overlap said edge, at leastpart of the developer mass is in electrical contact with the metalback-up plate, thereby short-circuiting the applied bias potential anddrawing sufiicient current to cause the bias voltage to dropsubstantially below the desired value.

A primary object of the present invention is to provide a novelapparatus and method which enable the electrophotographic sheet to bedeveloped by the ma netic brush technique along the marginal edge areasof the sheet while a bias potential is applied between the magneticbrush and the back-up plate, while preventing any substantial reductionin the bias voltage due to contact of the metal brush with the back-upplate.

In the drawings:

FIG. 1 is a schematic perspective view illustrating a first embodimentof the apparatus and method of the present invention;

FIG. 2 is a schematic perspective view illustrating a second embodimentof the invention;

FIG. 3 is a schematic perspective view illustrating a third embodimentof the invention;

FIG. 4- is a schematic perspective view illustrating a fourth embodimentof the invention;

FIG. 5 is a schematic sectional view of a system utilizing a rotarymagnetic brush and which may embody the invention in any of the fourforms thereof illustrated in FIGS. 1 to 4 inclusive; and

FIG. 6 is a sectional view taken substantially on line 6-6 of FIG. 5.

Referring first to FIG. 1, the reference numeral 11 designates generallya back-up plate adapted to contact the rear surface of theelectrophotographic sheet bearing on its front surface a latentelectrostatic image to be developed. The back-up plate 11 comprises asubstantially rectangular insulating plate or block 12 composed of anelectrical insulating material such as polystyrene, Lucite, Bakelite orsimilar phenol-formaldehyde resin, Wood or the like. Secured to orimbedded within the upper surface 13 of the insulating plate 12 are aplurality of rectangular conductive strips 14. The upper surfaces of thelatter may be coplanar With, or eX- tend above, the upper surface ofplate 12 so as to contact the rear surface of the electrophotographicsheet.

The strips 14 may be in the form of thin foil or thick plates and may becomposed of any suitable conductive metal such as copper, bronze oraluminum. The strips 14 are in parallel contiguous relation, and havetheir respective adjacent edges 15, 16 spaced from each other wherebyeach of the strips 14 is electrically insulated from the respectivestrips adjacent thereto.

The width of each conductive strip 4 is preferably about 2 inch, but maybe considerably greater as shown in the drawing for clarity inillustration. The width of each space between adjacent strips 14 ispreferably about .030 inch. In the event that the paper is movedlongitudinally of the backup plate 11 during the development step, asshown in the embodiment of FIGURES 5 and 6, the conductive strips 14 maybe oriented angularly with respect to the direction of paper movement;that is, diagonally with respect to the longitudinal axis of back-upplate 11 rather than transversely thereto as shown in FIGURE 1. Thisangular orientation of the strips 14 will eliminate any undesirableeffect of discontinuity due to the insulating spaces between the spacedstrips 14.

The reference numeral 17 designates a series of conventional resistorswhich may be of the carbon-deposited or Wire-wound or other suitabletype. Each of the resistors 17 is of a high resistance value, preferablyin the range between 100,000 ohms to 50 megohms. One end of each of theresistors 17 is connected by a lead 18 to a respective one of theconductive strips 14. The opposite ends of all the resistors 17 areconnected by loads 20 to a common lead 21 which is in turn connected tothe terminal 22 of a double-pole double-throw switch indicated generallyat 23. The other terminal 24 of the latter is connected by a lead 25 tothe magnet 26 of a magnetic brush 27 of any conventional or suitabletype. The switch 23 is also connected by leads 28, 29 to the terminals30, 31 of any suitable source of direct-current electric potentialindicated schematically by the symbol for a battery and designatedgenerally by the reference numeral 32.

It will thus be seen that upon closing of the double-pole double-throwswitch 23, the voltage of the electric potential source 32 will beapplied between the magnetic brush 2'7 and the conductive strips 14,with the resistors 17 in series between the respective conductive strips14 and one process.

of the terminals of the electric potential source 32. Upon reversing theswitch 23, the polarity of the applied bias potential will be reversed.The voltage between the terminals 30, 31 of the electric potentialsource 32 may be in the range of 50 to 1500 volts, with the range of 200to 800 volts being preferred for typical electrophotographic techniquessuch as those employed with the well-known zinc oxide resin-coatedpaper.

After the electrophotographic sheet has been exposed to a light image soas to form a latent electrostatic image on the front surface thereof,the sheet is placed with its rear surface against the back-up plate 11and in contact with the conductive metal strips 14. The switch 23 isclosed so as to apply the voltage of the electric potential source 32between the strips 14 and the magnetic brush 27. The latter may be ofeither the rotary. type shown in FIGURES 5 and 6 and to be described indetail below, or may be of the conventional manual type which is held inthe hand and manually applied during the development As the magneticbrush 27 moves over and in contact with the sheet front surface bearingthe latent electrostatic image, the developer powder or toner particles,electro-statically charged by triboelectric interaction with the ironfilings in the mass of developer mix 33 magnetically held to one end ofthe magnet 26, are wiped off and adhere to the charged areas of thefront surface of the electrophotographic sheet so as to form a visibleimage thereon.

It will be apparent that as the magnetic brush 27 is wiped over, orotherwise moves with respect to, the marginal areas of theelectrophotographic sheet so as to overlap an edge of the latter, aportion of the mass of developer mix 33 will contact the back-up plate11. If the latter were simply in the form of a unitary conductive metalplate, as heretofore employed in the art before the advent of thepresent invention, and if this metal plate were connected directly toone of the terminals 30, 31 0f the electric potential source 32 Withoutthe interposi tion of the series resistors 17, as has heretofore beenthe practice, it will be apparent that the potential source 32 would beshort-circuited at the point where the magnetic brush developer mass 33were to contact the back-up plate. This would cause a large current tobe drawn from the electric potential source 32 so as to reduce theapplied bias potential below the desired value, and the entire backupwould then be at the same potential as the brush.

However, in the present invention the conductive strips 14 are spacedand electrically insulated from each other and are connected to theelectric potential source 32 through relatively high value resistors 17.It will thus be seen that when the magnetic brush 27 overlaps an edge ofthe electrophotographic sheet, the developer mass 33 will contact one ora few of'the'conductive strips 14 and a short circuit of the potentialsource 32 will be prevented by reason of the fact that the resistors 17are in series with the conductive strips 14 and the potential source 32.As a result, the voltage of the applied bias potential is maintaind atsubstantially the same value during development of the marginal edgeareas' of the electrophotographic sheet as during development of theintermediate areas thereof. Furthermore, only those few conductivestrips 14 which actually contact the brush will be at zero potentialwith respect to the brush, and all of the other strips 14, constitutingpractically the entire area of the back-up plate 11 will be maintainedat the full bias potential with respect to the brush.

Referring to FIGURE 2 wherein is shown a second embodiment of theinvention, the reference numeral 11a indicates generally a back-up platehaving a somewhat different construction than the back-up plate 11 ofFIG- URE 1. The back-up plate 11a comprises a lower rectangular plate 34which is made of a highly conductive material such as copper, bronze oraluminum. Superimposed upon the conductive plate 34 is a plate 35composed of a high resistance semiconduetive material, such as any ofthe well-known conductive rubbers. As will be understood by thoseskilled in the art, the latter are rubber compositions to which aconductive material such as carbon black or superconductive carbon blackhas been added so as to enable the resistivity of the rubber to bereduced to a value as low as approximately ohms per cm. The resistivityof the conductive rubber employed for the semi-conductive plate 35should be preferably in the range from 10 to 10 ohms per cmfi. Insteadof conductive rubber, the plate 35 may be composed of any othersemi-conductive material having suitable mechanical characteristics anda resistivity in the above-noted range.

The term semi-conductive, as used throughout this specification and theappended claims, will be understood to refer to the range of electricalresistivity between approximately 1() ohms per cm. to approximately 10ohms per cmfi, in accordance with the accepted nomenclature asexemplified in the publication Battelle Technical Review, August 1957,page 9.

Secured to or embedded within the upper surface of the semi-conductiveplate 35 are a series of spaced contiguous conductive strips 14 whichmay be identical to those previously described with respect to theembodiment shown in FIGURE 1. The embodiment of FIGURE 2 also comprisesa double-pole double-throw switch 23, a direct-current source ofelectric potential 32 and a magnetic brush 27 arranged in the mannerdescribed above with respect to FIGURE 1. A lead 36 has one endconnected to the conductive metal plate 34 and its opposite end to theterminal 22 of the'switch 23.

It will be seen that a portion of the semi-conductive material of plate35 extends in series between the conductive plate 34 and each of theconductive strips 14. That is, each of the portions of thesemi-conductive material below a respective one of the strips 14 is ineffect a component having a substantial electrical resistance of a valuein the semi-conductive range. Each of these portions or components ofplate 35 has the equivalent effect of a respective one of the resistors17 of FIGURE 1.

When the magnetic brush 27 overlaps an edge of the electrophotographicsheet having its rear surface in contact with the upper surface ofback-up plate 11a", whereby a portion of the magnetic brush is in directcontact with one or two of the conductive strips 14, said portions orcomponents of the semi-conductive material of plate 35 will be in serieswith the by-pass circuit thus established around the edge of the sheet,so as to prevent said circuit from constituting a low-resistanceshortcircuit across the terminals 30, 31 of the electric potentialsource 32. This prevents the drawing of a large current which wouldcause the bias potential to be reduced below the desired value duringdevelopment of the marginal areas of the electrophotog'raphic sheet whenthe magnetic brush 27 overlaps the edges of the latter. Furthermore, asnoted above with respect to the embodiment of FIGURE 1, all of thestrips 14, with the exception of the one or few strips in contact withthe brush, are maintained at the desired potential with respect to thebrush.

Referring now to FIGURE 3, there is shown a third embodiment of theinvention wherein the reference numeral 11b indicates generally aback-up plate comprising a metal conductive plate 34 upon which issuperimposed a semi-conductive plate 37 composed of wear-resistantconductive rubber. The plate 37 of FIGURE 3 preferably has substantiallythe same electrical characteristics as the plate 35 of FIGURE 2, exceptthat the latter may be composed of soft rubber whereas plate 37 of thepresent embodiment is preferably composed of hard rubber re- 7 quiredfor wear-resistant characteristics. This is advantageous in the eventthat the embodiment shown in FIG- URE 3 is employed in an arrangementwherein the elec trophotographic sheet slides against the back-up plate11b during the development step in the manner shown in FIGURES 5 and 6.Since rubber. is normally a highly I insulating medium, the relativelylow resistivity (preferably in the range of ohms per cm. to 10 ohms percm?) of the material of plate 37 is provided by incorporating carbonblack or super-conductive carbon black therein in the manner Well-knownin the art.

Although the upper surface of plate 37 may be uniform and continuous, itcomprises in effect a plurality of contiguous adjoining areas eachcomposed of a material having an electrical conductivity in thesemi-conductive range as this range is normally defined in the art anddefined hereinabove. The component or portion of the material of plate37 immediately below each of said contiguous surface areas is in effecta component having a substantial electrical resistance of a value withinthe semiconductive range. That is, each of said components is in effectconnected in series between a respective one of said contiguous surfaceareas and the conductive late 34, whereby each component has theequivalent function of a respective one of the resistors 17 of theembodiment shown in FIGURE 1.

The conductive plate 34 of FIGURE 3 is connected by a lead 36 to adouble-pole double'throw switch 23 and associated with the latter are amagnetic brush 27 and a direct-current source of electrical potential 32in the manner described above with respect to the embodiments of FIGURESl and 2.

The mode of operation of the embodiment shown in FIGURE 3 is similar tothat of the embodiments of FIGURES l and 2. The electrophotographicsheet is placed with its rear surface in contact with the upper surfaceof the semi-conductive plate 37 and the magnetic brush 27 is wipedacross the sheet front surface bearing the electrostatic image. When thefibrous mass 33 composed of developer mix adhered to the magnet 26 ofthe magnetic brush 27 overlaps the edge of the electrophoto graphicsheet and contacts the upper surface of the semiconductive plate 37, alow-resistance short-circuit is prevented by reason of the fact that aportion or component of the material of plate 37 will be in seriesbetween the brush 27 and the conductive plate 34, thereby maintainingthe desired bias voltage during the development of the marginal areas ofthe electrophotographic sheet.

Referring now to FIGURE 4, there is shown a fourth embodiment of theinvention comprising a backup plate 110 comprising an insulating blockor plate 12 similar to the plate 12 of the embodiment shown in FIGURE 1.Mounted in the plate 12 are a plurality of circular conductive elements38. These may be in the form of thin circular discs secured to orimbedded within the upper surface 13 of plate 12, or they may be in theform of cylindrical plugs extending from the surface 13 down through theplate 12 to a substantial depth therein. The elements 38 may be composedof any highly conductive metal such as copper, bronze or aluminum, andmay be in the form of thin foil, or an electrolytic or otherwise coatedform of these metals. The conductive elements 38 are illustrated ascircular and are preferably of about one-eighth inch in diameter. Thecenter-to-center spacing of adjacent elements 38 is preferably about.180 to .200 inch.

There are provided a plurality of resistors 17, similar to thosedescribed above with respect to the embodiment of FIGURE 1, and each ofthe resistors 17 is connected by a lead 18 to a respective one of theconductive elements 38., The opposite end of each of the resistors 17 isconnected by a lead 20 to a conductive metal plate 39 which is in turnconnected by a lead 40 to the terminal 22 of a doublepole double-throwswitch 23 associated with a direct-current source of electricalpotential 32 and a magnetic brush 27 in the same manner described abovewith respect to the first three embodiments of the invention.

The mode of operation of the embodiment of FIGURE 4 will be apparentfrom the above description of the mode of operation of the embodimentshown in FIGURE 1, the conductive elements 38 being the equivalent andfunctioning in substantially the same manner as the conductive strips14. The conductive plate 39 merely serves as a common lead connectingone end of each of the resistors 17 of FIGURE 4 to the lead 40 which isin turn connected by the switch 23 to one of the terminals 30, 31 of thedirect-current electric potential source 32.

Referring now to FIGURES 5 and 6, there is shown an arrangement whereinany of the embodiments shown in FIGURES 1 to 4 inclusive may be utilizedwith a rotary magnetic brush. A longitudinal container or trough 42 isprovided with opposite end walls 43, 44 having openings therein forrotatably mounting a longitudinal shaft 45 formed of steel or any otherferromagnetic material. The

shaft is provided with a pair of shoulders or collars 46,

47 adapted to abut the exterior surfaces of the end walls 43, 44 so asto limit longitudinal movement of shaft 45. Mounted on the latter andfixedly secured thereto are a plurality of spaced parallel ellipticaldiscs 48 also composed of a ferromagnetic material.

One end 49 of shaft 45 is rotatably mounted within an opening formed inthe leg 50 of an L-shaped element 51 having another leg 52 to which issecured an element 53 of permanently magnetized material, such as Alnicoor the like. Secured to element 53 is a planar pole piece 54. The latteris adjacent to a back-up plate 11, but is spaced therefrom by an air gapindicated at 55. It will thus be seen that a continuous magnetic circuitis established from element 53 through pole piece 54, air gap 55,

back-up plate 11, discs 48, shaft 45, and then through L-shaped element51 to complete the circuit.

A gear 56 is fixedly secured to shaft 45 and is engaged by a pinion 57secured to the shaft 58 of a conventional electric motor 59, whereby thelatter may drivingly rotate the shaft 45.

The developer mix comprising toner particles and iron filings is placedwithin the trough 42 and is caused to adhere to the peripheries of thediscs 48, as indicated at 60, by means of the magnetic field. Theelectrophotographic sheet, indicated generally at 61 and comprising alayer of backing paper 62 having a coating of photoconductive material63 is fed vertically (as viewed in FIGURE 5) between the back-up plate11 and the elliptical discs 48 while the latter are rotated about theaxis of shaft 45 and through the developer mix in the trough 42. Thesheet 61 may be in the form of a continuous web wound in a roll, or arigid plate, or a single sheet of document size. The rear surface 64 ofthe electrophotographic sheet 61 contacts the back-up plate 11 and thefront surface comprising the photoconductive layer 63 is wiped by themasses of developer mix clinging to the peripheries of the rotatingdiscs 48.

In the event that the embodiment of the invention shown in FIGURE 1 isemployed in the type of apparatus shown in FIGURES 5 and 6, the back-upplate 11 will comprise a plurality of conductive strips 14 imbedded inthe surface of an insulating block 12 and in contact with the rearsurface 64 of sheet 61. Each of the conductive strips 14 is connected bya lead 18 to a respective one of a plurality of resistors 17 (only onebeing shown in FIG- URE 5 for simplicity in illustration). The oppositeend of each resistor 17 is connected by a common lead 21 to the terminal65 of the arm 66 of a potentiometer indicated generally at 67. Thelatter is connected to a double -pole double-throw switch 23 andelectric potential source 32 arranged as heretofore described. Oneterminal 68 of switch 23 is connected by a lead 69 to ground and shaft45 is also grounded by a lead 70 to complete the circuit for the biaspotential.

As the sheet 61 is moved with respect to the back-up plate 11 and therotating discs 48, the toner particles will be deposited on the chargedareas of the latter as indicated at 71, thereby forming a visible image.It will be understood that the rotary magnetic brush apparatus ofFIGURES 5 and 6 may be employed with the inventive embodiments ofFIGURES 2, 3 or 4 by substituting the 7 back-up plate 11a, l lb or 110respectively for the back-up plate 11 shown in FIGURES and 6.

The unidirectional field established between the magnetic brush and theback-up plate, as a result of the application of the bias potential,will add to or subtract from the electric field due to the latentelectrostatic image, depending upon the polarity of the applied biaspotential. When the two electric fields are properly additive, thephysical image formed by the deposited toner powder has a very highcontrast characteristic and a minimum amount of spurious deposit. If itis desired to reduce the contrast, the potentiometer 67 may be adjustedto reduce the voltage of the applied bias potential. If it is desired toproduce a reverse image during the development step, the switch 23 maybe reversed to cause the electric field of the applied bias potential tobuck or oppose the electric field of the latent electrostatic image.

Although the application of a bias potential to obtain these advantagesand modes of operation during development of the intermediate areas ofthe electrophotographic sheet, was known prior to the present invention,the novel apparatus and method disclosed and claimed herein enable thebias potential to be maintained at the desired voltage duringdevelopment of the marginal areas of the electrophotographic sheet.

The specific embodiments described in the above specification andillustrated in the accompanying drawings are merely illustrative ofseveral forms which the invention may take in practice, and there willreadily occur to those skilled in the art numerous modifications with-inthe scope of the invention as delineated by the appended claims whichare to be construed as broadly as possible in view of the prior art.

I claim:

1. An apparatus for developing an electrophotographic sheet having apreviously applied latent electrostatic image on the front surfacethereof, said apparatus comprising means adapted to contact the rearsurface of said sheet, said means comprising a plurality of contiguousareas ach composed of a material having an electrical conductivity of avalue at least in the semi-conductive range, and a plurality of spacedconductive elements disposed in spaced electrically insulated relationto each other and positioned adjacent said means adapted to contact therear surface of said sheet, a magnetic brush adapted to contact thefront surface of said sheet, a source of electrical potential having apair of terminals of opposite polarity, means connecting said magneticbrush to one of said terminals, and means connect-ingtsaid areas to theother of said terminals, said last-recited means having a substantialelectrical resistance of a value no greater than that of thesemi-conductive range. I

2. An apparatus as recited in claim 1 wherein said means having asubstantial electrical resistance comprises a plurality of resistors,each of said resistors being connected to a respective one of saidconductive elements,

each of said resistors being in series between its respective conductiveelement and said other terminal.

3. An apparatus as recited in claim 1 wherein said last-recited meanscomprises a plurality of components each having a substantial electricalresistance of a value no greater than that of the semi-conductive range,each of said components being connected to a respective one of saidareas, each of said components being in series between its respectivearea and said other terminal,

4. An apparatus as recited in claim 1 wherein said means adapted tocontact the rear surface of said sheet comprises a plate composed ofelectrically insulating material, each of said contiguous areascomprising a metallic conductive element, said conductive elements beingsecured to the surface of said plate and being in spaced configuousrelation to each other.

5. An apparatus as recited in claim 4 wherein said means having asubstantial electrical resistance comprises a plurality of resistorseach connected in series with a respective one of said conductiveelements.

6. An apparatus as recited in claim 1 and comprising a receptacle forcontaining a mixture of toner particles and iron filings, means mountingsaid magnetic brush for rotation in contact with said mixture, saidmeans adapted to contact the rear surface of said sheet being spacedfrom said 'magnetic brush whereby the electro-photographic sheet maypass through the space therebetween,

and drive means for rotating said brush to cause the latter to conveysaid mixture from the receptacle to said front surface of the sheet.

7. An apparatus as recited in claim 6 wherein said means adapted tocontact the rear surface of said sheet comprises a plate composed ofelectrically insulating material, each of said contiguous areascomprising a metallic conductive element, said conductive elements beingsecured to the surface of said plate and being in spaced contiguousinsulated relation to each otherr 8. An apparatus as recited in claim 7wherein said means having a substantial electrical resistance comprisesa plurality of resistors, each of said resistors being connected to arespective one of said conductive elements, each of said resistors beingin series between its respective conductive element and said otherterminal of said source of electrical potential.

References Cited in the file of this patent UNITED STATES PATENTS YoungMar. 26, 1957 Giaimo June 16,

and

